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This week we have had three different wheel speed sensor faults with interesting problems.  All of which needed some detailed testing to resolve complications:

The first was a Volkswagen Passat – The car came to us with a bit of history as another garage had looked at the car first and read a fault code which said “wheel speed sensor LHF incoherent”. They had tried fitting a wheel speed sensor but the fault was still present.  They then tried fitting a wheel bearing magnetic ring (which the sensor points at) this also didn’t work.  Then they called in the services of a mobile electrician who tested the wiring and suggested it must be the control module.  Luckily the garage didn’t replace the control module but they did remove it from the car and sent it for bench testing by a specialist (quite a significant job) but it came back “no fault found”. At this point the garage had exhausted all of their options and suggested that the customer visit Pinpoint Diagnostics.

Using our technical knowledge and an oscilloscope we were able to test the system accurately and prove beyond doubt that the wheel speed sensor was indeed faulty.  The customer told us the original part had been refitted when the garage had not been able to find the fault. It turned out that the original fault had been the sensor but the replacement part the garage used was not of sufficient quality to return the signal correctly and the car was therefore still recording a fault with the new sensor.  After our testing we were confident that a genuine part would fix the issue and after discussing and agreeing this with the customer we fitted a genuine Volkswagen sensor and the fault was fixed. Had the previous garage been able to test the sensor accurately with an oscilloscope, they would have identified the faulty part and saved themselves and their customer lots of time and money.

A few days later a Mercedes C Class arrived also having had a wheel speed sensor fitted by a previous garage. This time the garage didn’t go any further but recognising that they needed more information they enlisted our help to test the car. We were able to establish that the vehicle did in fact need a sensor, but again the aftermarket part that had been fitted was faulty.   A good quality part was fitted and the problem was resolved.  Interestingly the car had several warning lights showing for the ABS, traction control, cruise control and tyre pressure monitoring. The car also had a lack of power (limp home mode had been initiated) all due to one ABS sensor fault. All of this was resolved by fitting the quality part.

Both of these jobs illustrate the issue that a plug in diagnostic test does not provide enough information to isolate a fault and repair it correctly.  Even if the parts are cheap enough to try replacing them without testing, if you can’t prove that the original part is definitely faulty and that the new part does actually work you could just be adding a whole extra layer of complication.

Please note that most aftermarket parts are fit for purpose.  We often use good quality aftermarket parts as they can save money and provide good value for our customers, but we are always wary of very cheap parts as they can sometimes introduce new faults or not work reliably, as demonstrated above. Cheapest is often not the best option when it comes to car parts.

The final wheel speed sensor job of the week was a Jaguar X Type.  Again this job came to us with a bit of history. Two years ago the customer had come to us when his car had failed its MOT on the ABS light. He had struggled to find anyone in the local area to look at his car because it was a Jaguar and not many garages have the right diagnostic tools. We have the dealer level tool and the customer was recommend to Pinpoint Diagnostics by a local garage. At this time we tested the wheel speed sensors and proved conclusively that the right hand front sensor was faulty. The part was replaced and the light was reset.

Twelve months later (one year ago) the customer called again to say that the car had been ok, but now, just before his MOT the light had come back on.  We tested all of the sensors again and this time it was the left hand front sensor that needed replaced.

This week, almost exactly twelve months later again, just before his MOT, the light came on again.  We checked our customer records, read the fault code and we were surprised when the code indicated that the left hand front wheel speed sensor was faulty.  The sensor we fitted last year had been a quality aftermarket part (to save the customer money) and now, after the previous wheel speed sensor jobs this week, I was wondering if the part had failed.  However, with our oscilloscope and multi-meter testing, we were able to check the sensor and determine that it was functioning correctly and instead the wiring to the sensor had failed in an area under the wing.

Had we not had the ability to test the sensor it would have been impossible to complete this job without wasting money on a replacement part. Our tests located the faulty wiring, we repaired it, no parts required, and the customers car passed its MOT once again.  We are all hoping that we don’t need to do the same again next year. However there are another 2 twelve year old sensors on the car that have not yet been replaced, so you never know!